RESEARCH NEWS

December 2004 25

Better understanding of chemical

structure/physical property correlations

in organic semiconducting materials,

particularly the interactions of

chromophores on a microscopic level,

could aid in designing improved

materials for light-emitting diodes

(LEDs), solar cells, photodiodes, and

other optoelectronic devices. Secondary

and tertiary structure, such as chain

folding, can be as important as the

primary chemical structure of the

polymer in determining their optical

properties. Now researchers from

Germany’s Ludwig-Maximilians-

Universität and Universität Wuppertal

have found that structure-property

correlations in conjugated polymers

derive mainly from chain morphology

rather than chromophoric properties

[Schindler et al., Proc. Natl. Acad. Sci.

USA (2004) 101, 14695].

John M. Lupton and coworkers used

single-molecule spectroscopy to study

comparatively ordered and highly

disordered polymers. The technique

enables the researchers to identify

single chromophores in highly ordered

polymers and demonstrate that the

efficiency of intramolecular excitation

energy transfer is dominated by the

temperature-dependent spectral

overlap of chromophores. Even though

the ordered and disordered polymers

exhibit virtually identical spectroscopic

features on the single-chromophore

level, the single-molecule properties of

the two conjugated polymers are quite

different. The energetic range of

spectral diffusion is small but can

influence the intramolecular excitation

energy transfer and the emission. The

fundamental difference between

ordered and disordered polymers is

that the latter can undergo coherent

interchromophoric coupling resulting in

strong spectral broadening.

John K. Borchardt

Polymer folding propertiesPOLYMERS

Patterning polymers is more efficientPOLYMERS

Light can be trapped in an organic light-emitting diode (LED)as a result of total internal reflection. As much as 70% ofthe photons produced by a light-emitting polymer (LEP) with arefractive index greater than two can be trapped in the glasssubstrate or in the polymer and indium tin oxide (ITO) anodelayers. Preventing this trapping would significantly increaseLED power efficiency. Current methods of accomplishing thisinclude the use of microlenses, aerogel layers, silicamicrospheres, or periodic corrugations fabricated by spin-coating an LEP on top of a sinusoidal photoresist layer andhot-embossing the layer with a patterned stamp. However,these methods are limited by excessive light absorption bythe photoresist and detrimental thermal effects. Researchers from the University of Cambridge, UK haveovercome these difficulties by developing a method thatharnesses the waveguide modes trapped in polymer-blendLEDs [Corcoran et al., Appl. Phys. Lett. (2004) 85 (14),2965]. Richard H. Friend and coworkers accomplished this byfabricating self-organized, two-dimensional micron-scalephotonic structures within the emissive layer of a polymer-blend LED. Fabrication involves phase separation of twosemiconducting polymers directed by a surface chemicalpattern to form a relief-and-phase grating. Phase separationin polymer blends is strongly dependent upon the substratesurface energy. On patterned surfaces, preferentialsegregation of one of the blend components to higher surfaceenergy regions results in periodic domain structures. TheCambridge team spin-cast a 100 nm thick film of poly(9,9'-dioctylfluorene-co-benzothiadiazole), or F8BT, and poly(9,9'-dioctylfluorene), or PFO, onto a two-dimensional patternedanode and allowed them to phase separate. Micro-contactprinting is then used to create a pattern of 2 µm dots with aperiodicity of 4 µm. “By utilizing surface patterning and self-organization, we were able to construct a photonic structurefrom the actual polymers generating the light, therebyimproving the device efficiency two-fold without changing any

of the device layout,” explains Wilhelm T. S. Huck. While this does not lead to efficient photonic gratingstructures for visible light, these phase-separated domainsdevelop selectively amplified Fourier harmonics locked todomain boundaries. The result of these high-order harmonicsis strong broadband outcoupling of the waveguide modeswithout causing spectral dispersion. This can double LEDexternal quantum and power efficiencies while maintainingspectral integrity with viewing angle. “Since the surface patterning is compatible with devicefabrication and self-organization in polymer blends is auniversal phenomena, we envisage that this strategy can beused to improve device efficiency in all polymer LEDs,” saysHuck. The researchers are now working on getting a betterunderstanding of how to control the patterns in morecomplex blends and extend the scope to the nanoscale.John K. Borchardt

Green and blue fluorescence microscopy images of 100 nm films clearly showing the

phase separation process. (Reproduced with permission from Adv. Mater. © 2004 Wiley-

VCH Verlag.)